On the formation of ship tracks

On of the interesting manmade climate phenomenon is that of Ship Tracks, whereby a ship, with all the exhaust particles its releases, can change the characteristics of clouds in its wake.

The physics behind the effect is relatively simple. To form clouds, the water vapor needs cloud condensation nuclei to condense upon. Without such particles, one can reach and surpass 100% humidity without getting condensation (I often joke that it would feel like DC in summer... which of course is wrong since there is enough pollution there to have as many cloud condensation nuclei as the water vapor could wish).

Over the oceans, such particles are not overly abundant. This means that a passing ship could significantly increase the local density of cloud condensation nuclei. This effect is of course amplified by the fact that ships tend to burn the cheapest, most polluting fuel they can find. They're in international waters after all.

In the regions with the larger particle density, the same amount of condensing water vapor now has more particles to condense upon. This means that the clouds are then formed from more numerous but smaller droplets. Overall, we therefore get a much higher surface to volume ratio. Since the volume is the same (the given amount of condensing vapor), we get a larger total reflecting surface area. These clouds are therefore whiter. They also tend to live longer because it is harder for those smaller drops to grow in size, and rain out. The net result is that the pollution wakes of ships can manifest themselves as a white streak in the clouds, as is apparent in the first figure.

Last week I was on such a polluting boat, and to my amazement, one could actually see the cloud of dark polluting particles linger behind the boat for miles. The ship definitely polluted like hell!

The wake of a large boat, traveling at 14 knots westward. Somewhat north of the wake (i.e., to the left) one can see a gray semi-circle smog left by the boat, which was pushed northwards by a constant southerly wind.

I didn't see any effect on clouds, however, I wasn't in the pristine pacific air after all. It was the eastern mediterranean somewhere south of Cyprus, with many land originating pollutants, dust particles, etc. So I am not sure whether meaningful ship tracks can actually form there (Image Google'ing Mediterranean Ship Tracks doesn't pop up any ships tracks from this sea). Also, I am not sure that the ship tracks are actually apparent from the ships producing them, that is, if you were to stand on one of the ships producing the tracks in the first figure, would you be able to see the "track" in the clouds? Any sailor out there who can comment?

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Comments (7)

Which atoms and particles can sreve as condensation nuclei. I am envisioning it like a chemical recrystalization, in that anything the water vapor can adhere to and condense near would serve...is this correct?

Aug 30, 2007

shaviv

Water needs condensation nuclei because of the energy associated with the surface tension of a small droplet. Because of this energy, there is a high potential barrier which doesn't let vapor condense, just like that, even if the humidity is above 100%.

To bypass this barrier, the water vapor can start condensing on an already existing surface (such that forming this surface doesn't cost energy). This surface is that of a cloud condensation nucleus.

Different particles can serve as those nuclei. You can have dust or pollution particles, you can have small salt particles, or, you can actually grow those particles if there are no significant natural sources.

Such growth can take place over the oceans, where a cluster of a few sulphuric acid and water molecules start adhering to them more water molecules.

Sep 04, 2007

Demesure (not verified)

Dr Shaviv,
I am a windboat lover and I have crossed the Atlantic 4x on small boats (less than 20 m long). We depart from France, down to the Canaries then go straight to the Antilles sailing by wind. Well it's at snail speed compared to Al Gore's private jetting, but hey, that's my bit to reduce my carbon footprint and "save the planet".

Back to the topic, each time, we see huge cargos, sometimes on high seas and clear sky. I don't remember seeing they have cloud trails or I may confound those with exhaust pollution and don't remember having heard of such descriptions (but I confess we have so many topics to talk about in sailor bars).
Now you have said that, I'll watch cargos passing in pollution free areas with quite different eyes.

Sep 04, 2007

shaviv

You have my envy. I would of course have preferred sailing a boat than taking a cruise ship, but, was forced to adhere to uncontrollable constraints.

Anyway, I don't know your background, but as a scientist, I can never see something without thinking about the science behind it. Often when you understand something, it is more beautiful.

Cheers.
-- Nir

Sep 04, 2007

Aika (not verified)

I think that noticing the ship track from the sea-level may be difficult. They do not look like plane contrails - white lines in a blue sky. These are whiter lines on white background. They are also wide - the engine exhausts tend to spread horizontally and we (would) see ship tracks from a much smaller distance than contrails.
It would be interesting to find, in what distance to the ship the track becomes visible - what are the relations of the ship speed, the exhaust vertical velocity and the condensation rate.

Nov 10, 2008

David Halliday (not verified)

I recall having read some on these ship tracks a number of months ago (maybe a year?). As I recall, they don't form immediately behind the ships, but some miles behind. Apparently the reactions involved take time: Time to garner sufficient water vapor; perhaps time to reach the proper height; and, possibly, even time to form the right particulates (the particulates you saw are likely organic and/or elemental carbon particles [soot]---which are not particularly hydrophilic---sulfuric acid and/or sulfates form later from the SO2).

Just some added info for the discussion.

P.S. As a physicist, working now in the environmental field, I find your theory of cosmic ray "catalyzed" cloud nucleation quite intriguing.

Sep 20, 2007

geremy

What an interesting information, ship tracks have a hypnotic power on me, I can sit and watch them for hours in the blue ocean water. That was my favorite waste of time in my last Royal Caribbean cruise, so now I learn they have a different shape according to different ships.